It is commonly recognized in the operation of spark ignited engines that the size of the gap between the electrode of a spark plug and the adjacent ground strap is critical. It is also recognized that the size of the gap required to provide excellent starting and low idle characteristics is far different from the size of the gap required for excellent engine performance at high loads. It is therefore common practice to provide a spark gap that will provide acceptable operation under all conditions, but that is ideal to none. Primary consideration, however, is given to a gap size that will at least provide good starting characteristics.
The spark gap selected to provide good starting characteristics is much larger than is required to operate the engine under high load conditions. This requires a relatively high voltage to provide a spark that will travel across the gap under high engine loads. The high voltage tends to allow the spark to pull material away from the electrode as it jumps from the electrode only to deposit that material on the ground strap. This continual erosion of the electrode and buildup of material on the ground strap tends to shorten the life of the spark plug dramatically.
One known method of controlling the buildup of material on the tip of the plug is disclosed in U.S. Pat. No. 4,539,503, issued to Friedrich Esper et al. on Sep. 3, 1985. This patent discloses a spark plug that utilizes an electrode that is encapsulated in an insulator. The material of both the electrode and insulator is such that the operating temperature of the plug dictates the heat transfer between the two components. In doing so, the deposits on the insulator will be burned off while keeping the insulator below a temperature that would cause glow ignition. In combination with very specific materials of each component, they are designed such that a gap exists between the electrode and the insulator under low temperature operation. As the temperature increases the metal of the electrode will "flow" to fill the gap and alter the heat transfer therebetween. While there is some physical adjustment between the electrode and the ground strap, the effective gap that is exposed to the combustible mixture in the combustion chamber, remains the same because of the encapsulation of the electrode within the insulator. The distance between the insulator and the ground strap doesn't change and the operation of spark plug is unaltered.
Another spark plug design that utilizes an adjustable gap is disclosed in U.S. Pat. No. 3,612,931, issued on Oct. 12, 1971, and U.S. Pat. No. 3,743,877, issued on Jul. 3, 1973, both of which are issued to William P. Strumbos. These patents disclose the use of a heat shunt that has a thermal gap positioned between the shunt and the outer shell. The gap prevents heat transfer through the shunt at lower operating temperatures and the shunt will expand at higher operating temperatures to bridge the gap to provide improved cooling of the plug. The adjustment of the air gap is solely intended to alter the transmission of heat with respect to the plug and does not alter the characteristics of the spark between the electrode and the ground member.
The present invention is directed to overcoming one or more of the problems as set forth above.